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Description
Recent weak-lensing measurements reported that circular velocity curves of galaxies remain flat out to ~1 Mpc, potentially posing a tension with the standard ΛCDM model. In this work, we investigate the origin of these extended flat rotation curves using the IllustrisTNG cosmological simulation. We select isolated galaxies from the simulation and construct circular velocity profiles in three distinctive ways: (1) directly from the enclosed mass of particles gravitationally bound to the host galaxy, (2) from the enclosed mass of all particles within a large aperture, and (3) from the excess surface density profile measured in the simulation, which is designed to most closely mirror what is measured in weak-lensing observations. We find that ΛCDM can naturally produce flat rotation curves out to the virial radius and, in most mass bins, nearly flat profiles out to 1 Mpc when the contribution from surrounding matter is included. This indicates that the extended flatness inferred from weak-lensing can be explained largely by environmental mass contributions, rather than by a failure of ΛCDM itself. The most notable residual discrepancy arises in the lowest-mass bin, where the simulated profile declines beyond the virial radius while the observed profile remains flat to 1 Mpc. We further find that, in this mass regime, galaxies residing in more massive and lower-concentration halos tend to exhibit flatter outer rotation curves.